Discontinuous clamping wrenches are tools widely used in the industrial sector, particularly in the field of motor vehicle manufacture.
The principle of these wrenches consists of generating torque pulses, which are transmitted to the screw to be tightened.
These pulses are themselves generated by a system comprising:                a pneumatic motor;        a pulse clutch;        an output shaft;        a casing incorporating the above-mentioned items and wherein a part forms a handle.        
The pulse clutch operates cyclically as follows:                on approximately 170°, the clutch is disengaged and allows the motor to accelerate freely and accumulate kinetic energy;        on the next 10°, the clutch is engaged and transmits the kinetic energy contained in the rotor of the pneumatic motor to the screw via the output shaft; this kinetic energy is converted into a brief but high-amplitude torque pulse;        the same cycle is repeated for every subsequent 180° (a period other than 180° may however be envisaged).        
Discontinuous clamping wrenches offer the major advantage of enabling clamping at a high torque level with a low reaction torque in the operator's hand.
These tools enable a short clamping cycle time compared to clamping tools applying a continuous rotation movement to the screw until the desired torque level is obtained.
The major drawback is that it is difficult to monitor the effective torque applied in the screw.
It is noted that the term effective torque refers to the torque that would be applied by a torque wrench in a static manner or at a slow speed.
Clamping level monitoring systems have been proposed by the prior art.
In practice, experienced users of impact wrenches know, from experience, how to detect, through the touch sensation provided by the casing during clamping, when the required clamping level has been reached.
In other words, simply holding the tool and sensing the force of the tool provided to the operator via the casing enable the operator to evaluate whether the required clamping level has been reached or not.
However, this practice is based on an interpretation by the operator of the sensations provided by the tool.
Therefore, incorrect results, due to uncertain perception of the clamping level, are frequently observed.
In order to remedy this drawback, clamping level monitoring systems have been proposed by the prior art.
Discontinuous clamping wrenches are known wherein a torque sensor is positioned on the output shaft so as to measure the torque pulses. A solution of this type is described in the patent document published under the number JP-4 115 877 (FIG. 1).
Discontinuous clamping wrenches are also known wherein an angle sensor measures the deceleration of the assembly formed by the pneumatic motor rotor and the clutch casing.
The principle of this solution lies in that the rotoric inertia and the deceleration of the rotor/casing assembly during the torque pulse being known, it is possible to calculate the amplitude thereof.
Such a solution is particularly described in the patent document published under the number WO-2005/05390 (FIG. 2).
In both solutions described above, the torque pulses are then processed by an algorithm to determine whether the required clamping level has been reached.
The major drawback of these technologies is that they require the integration of sensors on the rotating parts, resulting in:                assembly and/or setting difficulties;        dependency with respect to wear parts liable to induce, in the long term, a reliability defect;        complex maintenance operations.        